The invention relates to a process for producing polymer-graphite nanocomposites, polymer-graphite nanocomposites which can be produced by this process and also the shaped bodies, semifinished parts, films, fibers and foams produced from the polymer-graphite nanocomposites. According to the invention, the graphite is oxidized in the first step of the process, then converted into an aqueous dispersion and mixed with an aqueous polymer dispersion. The polymer-graphite mixture is subsequently separated off from the aqueous phase. During the course of the process, the graphite oxide is reduced again to graphite in an intermediate step. The polymer-graphite nanocomposites produced according to the invention have improved mechanical properties compared to polymer-graphite nanocomposites produced according to the prior art.
Polymers reinforced with fillers have been known for a long time. The intention of the fillers is, in particular, to improve the mechanical properties and the thermal and electrical conductivity of the polymers. Fillers used are various materials such as wood fibers and glass fibers, alumina, carbon black, graphite and carbon nanotubes (CNTs) (see, for example, H.-G. Elias, Macromolecules Volume 4, page 372, Wiley-VCH Verlag Weinheim 2009). Polymers which comprise particles or fibers having a size in at least one dimension below about 100 nm as filler are also referred to as polymer nanocomposites. Fillers which are particularly suitable for this purpose are, for example, alumina platelets, graphite and graphite oxide platelets, carbon nanotubes and silicate nanoplatelets. Polymer nanocomposites display a further improvement in their mechanical properties compared to polymers reinforced with coarser fillers. Carbon nanotubes or graphite as nanosize fillers can, when they are used in the appropriate concentrations, make the polymer nanocomposite conductive, with graphite having a significant cost advantage over carbon nanotubes.
One difficulty in the production of polymer nanocomposites is dispersion of the nanoparticles in the polymer, since they should be as uniformly and finely distributed therein as possible. To produce a polymer-graphite nanocomposite, the graphite which has a layer structure should firstly be converted into nanoparticles or be brought into a form which allows conversion of the graphite into nanoparticles during mixing with the polymer. The graphite nanoparticles are subsequently mixed with the polymer.
US 2006/0231792 A1 discloses firstly expanding graphite by heating by means of microwaves or radio waves and comminuting the platelets formed to a length below 200 μm. The graphite platelets are subsequently mixed with polymers such as polyamides, polyolefins and polycarbonate in a blender.
S. Stankovic et. al., Nature 2006, Vol. 442, pages 282-286, disclose graphite-based composites which are produced by mixing a solution of polystyrene and graphite oxide platelets which have been treated with vinyl isocyanate in methylformamide and subsequently reducing the graphite oxide by means of dimethylhydrazine. The polystyrene-graphite nanocomposite is separated off by precipitation in methanol. A disadvantage of this process is the treatment of the graphite oxide platelets with vinyl isocyanate, which represents an additional process step and also introduces an additional component into the composite.
S. Stankovic et. al., J. Mater Chem. 2006, 16, pages 155-158, describe the production of stable, aqueous dispersions of reduced graphite oxide nanoplatelets, which is, as a result of the presence of an anionic polymer such as the sodium salt of poly-4-styrenesulfonic acid, made possible in the reduction in the aqueous phase. Here, the reduced graphite oxide nanoplatelets are coated with the anionic polymer.
P. Steuer et. al., Macromol. Rapid Commun. 2009, 30, pages 316-327, describe polymer-graphite nanocomposites which comprise firstly oxidized and subsequently thermally reduced graphite. The composites are obtained by mixing the thermally reduced graphite with a solution of the respective polymer or by dispersing the thermally reduced graphite in acetone and adding powdered polyamide, and subsequently drying and compounding the mixture.
Despite the processes known from the prior art for producing polymer-graphite nanocomposites, there is a need to produce polymer-graphite nanocomposites which have further improved properties, in whose production as few as possible additional components are introduced and which are very simple to produce.